This invention relates to nuclear reactors and it has particular relationship to such processes as the refueling of nuclear reactors. Typically, a nuclear reactor includes a plurality of fuel assemblies. Each fuel assembly includes a plurality of fuel elements. In addition, a fuel assembly includes a plurality of thimbles in which control rods and coolant-displacement rods are removably inserted. Coolant-displacement rods are sometimes referred to as moderator rods. These control and moderator rods will sometimes be referred to herein together as penetrating rods because they penetrate into a fuel assembly. Each penetrating rod is connected to a drive rod which is operated to insert or remove the penetrating rod from its thimble. Typically, there may be as many as 185 sets of drive and penetrating rods for each fuel assembly.
The practice in accordance with the teachings of the prior art for removing a fuel assembly during refueling or for other purposes has been to remove the fuel assembly as a whole including its penetrating rods. This practice demands that each drive rod be disconnected from its associated penetrating rod. This disconnecting operation is time-consuming and costly particularly as the drive rods and penetrating rods are radioactively contaminated and are under water and the disconnection must be carried out with long-handled tools operated from a platform 30 or 40 feet above the reactor. To overcome this drawback, it is desirable that the transfer of each fuel assembly during refueling or the like be an unrodded transfer. The formidable task of disconnecting the drive rods from their associated penetrating rods is by this practice dispensed with. Preliminary to the transfer of a fuel assembly, the drive rods with the penetrating rods suspended from them are removed as a unit form the reactor and the transfer proceeds without the penetrating rods. After a fuel assembly is replaced, the drive and penetrating rods are replaced in the reactor and the penetrating rods are inserted in the thimbles of the replaced fuel assembly.
It is an object of this invention to provide a method for removing and replacing as a unit the drive rods and the associated penetrating rods of a fuel assembly from a reactor during a refueling operation or the like. It is also an object of the invention to provide apparatus uniquely suitable for practicing this method. Another object of this invention is to provide such a method and such apparatus in whose practice and use excessive loading of a drive rod during a removal operation or the excessive unloading of a drive rod during a replacement operation, such as might occur when a penetration rod encounters an obstruction, shall be detectable.